Molecular imaging is an emerging technology that allows for visualization of interactions between molecular probes and biological targets. Positron emission tomography (PET), micro-PET and PET/CT, are state-of-the-art nuclear medicine imaging modalities, which use nano- to picomolar concentrations of the corresponding probes (radiotracers) to achieve images of biological processes within the living system. Selection of the proper radionuclide and synthetic approach for radiotracer design are critical. Positron-emitting isotopes frequently used include 11C and 18F. One non-traditional PET radionuclide, 64Cu, shows promise as both a suitable PET imaging and therapeutic radionuclide due to its nuclear characteristics (T1/2=12.7 h, β+: 17.4%, Eβ+max=656 keV; β−: 39%, Eβ−max=573 keV), and the availability of its large-scale production with high specific activity.
Stable attachment of radioactive 64Cu2+ to targeted imaging probes requires the use of a bifunctional chelator (BFC), which is used to connect a radionuclide and bioactive molecule to form the 64Cu-radiopharmaceutical. Extensive efforts have been devoted to the development of BFCs for 64Cu labeling. Three of the most common chelators studied have been the macrocyclic ligands DOTA (1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″-tetraacetic acid), TETA (1,4,8,11-tetraazacyclotetradecane-N,N′,N″,N′″-tetraacetic acid), and cross-bridged tetraamine ligands. (Refs. 1-3) However, dissociation of 64Cu from the BFC in vivo and harsh labeling conditions (e.g., incubation at 75° C. under basic conditions) impair the use of these chelators in preparing biomolecule-based 64Cu-radiopharmaceuticals. (Ref. 4) The BFC DOTA has been used for 64Cu2+ labeling. (Refs. 3, 5, 6) However, the limited stability of the copper chelate in vivo has hindered its application. (Ref. 37)
The integrin αvβ3 receptor has been the attractive target of intensive research given its major role in several distinct processes, such as tumor angiogenesis and metastasis, and osteoclast mediated bone resorption. (Refs. 7, 8) The molecular imaging of integrin αvβ3 expression will allow the detection of cancer and other angiogenesis related diseases, patient stratification, and treatment monitoring of anti-angiogenesis based therapy. (Refs. 9, 10) Although we and others have successfully developed various DOTA conjugated RGD peptides for multimodality imaging of integrin αvβ3 expression. (Refs. 9, 10, 11, 31), the loss of 64Cu from the chelator has lead to unfavorable high retention in liver, resulting in high background. Therefore, the choice of a more stable BFC is preferred.
Although some other novel BFCs have been developed and shown promise for use in copper-64 labeling (Refs. 2, 4, 12, 32), there is lack of adequate published data regarding the biological activity of these complexes. Some have high uptake in lung, liver and muscle, which may impair the detection of small lesions in the chest or/and abdominal regions. (Refs. 2, 4, 12, 32)
Recently, Sargeson and co-workers reported a new type BFC based on the sarcophagine (3, 6, 10, 13, 16,19-hexaazabicyclo[6.6.6]icosane, Sar, FIG. 1. A-2) for preparation of 64Cu-radiopharmaceuticals, (Refs. 2, 36) These ligands coordinate 64Cu2+ within the multiple macrocyclic rings comprising the Sar cage structure, yielding stable complexes that are inert to the dissociation of the metal ion.
The caged-like BFC Sar ligands are able to selectively label 64Cu2+ rapidly over a wider range of pH value under mild conditions. (Ref. 2) However, there are only a few reports that describe the complexation, stability and biodistribution of the 64Cu complexes of the Sar ligand, and only (NH2)2-Sar (Diamsar) and 1-N-(4-aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane-1,8-diamine (SarAr) have been reported as a BFC for the development of 64Cu-radiopharmaceuticals. (Refs. 2, 13, 35, 36) Moreover, the relatively nontrivial and multi-step synthesis of Sar ligands may limit their future applications. (Ref. 4) However, the SARAR BFC utilizes the C-terminal for carboxylic acid for conjugation. This may be a disadvantage as the C-terminus is often found to be a crucial part for maintaining biological activity.
As such, there is a need for improved bifunctional chelators that can efficiently form stable complexes with metals, including Copper-64 under mild conditions.
There is also a need for new radiopharmaceuticals and imaging agents that are stable in vivo for imaging and other.
There is also a need for simplified methods for making bifunctional chelators.